2) The only reason to not use the 12 gauge is that it costs more (pennies) and is harder to work with (barely).
The nice thing about using the 12 ga is it is protected by a 20 amp breaker, and thus can stand up to a short better than the 14 ga.
You DW could JUST be serviced by a 15 amp breaker. There is debate as to whether it is safer at 15 or at 20.
Let me ask you this: are the breakers for your dishwasher and your garbage disposal tied together? If they are two circuits running to one duplex plug, they should be protected by a two pole (not a tandem) breaker, like you would use to protect a 240v 20 amp circuit. That way, if one circuit trips the breaker, both breakers trip and all the wires going to the receptacle go cold together. Which is much safer.
If they are all wired as a three wire circuit (two hots sharing a neutral, and the hots being fed from the two opposite legs of the sine wave) then everything is clean. This could be conduit with wire fished into it. Or it could be 12/3 non-metalic. Black, red, white, bare. Or the same thing in metal clad.
If, however, the neutrals and hots for two circuits take different paths, it is important to use both neutrals. Break the tab on the receptacle for the two neutrals, and don't confuse the one with the other, ensure that it is with its correct hot. I tape things together in circumstances like that, black and red. Almost any electrician following me would recognize my purpose.